Engine combustion using gasoline fuel may generate particulate matter (PM) (such as soot and aerosols) that may be exhausted to the atmosphere. To enable emissions compliance, gasoline particulate filters (GPF) may be included in the engine exhaust, to filter out exhaust PMs before releasing the exhaust to the atmosphere. A higher efficiency GPF comprising a denser filtration mesh may be used for increased emissions quality, especially during cold-start conditions.
To maintain the efficiency of the exhaust system particulate filter, the filter may need to be intermittently regenerated. In one example approach, shown by Neely et al. in U.S. Pat. No. 9,027,333, a diesel particulate filter (DPF) is regenerated responsive to a higher than threshold soot level. In particular, the regeneration is controlled so that a small level of soot is maintained on the filter in order to provide optimal efficiency for hydrocarbon conversion during conditions such as an upcoming cold start. In other approaches, the entire soot accumulated on the filter may be removed during the regeneration process.
However, the inventors herein have recognized potential issues with such approaches. As one example, since filters tend to be least efficient when they are clean of soot or ash, filters with higher filtration features (such as a denser mesh rate) are typically implemented in engine systems. When the filters are clean, the pores in the substrate may be fully open, consequently the particles may be able to traverse more easily through them and the probability of collisions and sticking may be reduced, thereby adversely affecting the soot capture rate. However, use of higher filtration capability filters may result in elevated exhaust backpressure which may adversely affect engine power and increase fuel consumption. Also, such filters can add significant costs. Another issue is that the optimal level of residual soot level on the filter may vary with operating conditions. For example, the residual soot level which corresponds to optimal emissions control during a cold-start may be higher than the residual soot level that is optimal for idling engine conditions. As a result, the soot level remaining on the filter following a regeneration at an engine cold-start may lead to inefficient exhaust emissions during a subsequent engine idling condition. Further still, the level of ash accumulated on the filter following a regeneration event, as well as the distribution of the ash throughout the filter, may influence operation of the filter as well as the resulting exhaust backpressure. For example, even if the residual soot level is lower, if there is a significant amount of ash left over in the filter from the previous regeneration event, the total loading on the filter may be higher than the optimal soot level desired for improved emissions quality.
In one example, the issues described above may be addressed by a method comprising, responsive to actual soot level at an exhaust particulate filter being lower than a target soot level, adjusting one or more of a fuel injection timing and a fuel injection pressure to increase soot output of the engine until the actual soot level is at the target soot level, the target soot level varied based on engine temperature and engine load. In this way, a lower filtration capability GPF may be utilized to achieve a lower backpressure by actively maintaining a level of residual soot on the filter.
As one example, a gasoline particulate filter (GPF) with a lower filtration feature (such as a lower density filtration mesh) may be coupled to an engine exhaust system. An optimal residual soot level (target level) to be maintained at the GPF may be determined by an engine controller based on engine operating conditions including engine temperature, engine speed, engine load, fueling schedule etc. A soot level at the GPF may be estimated based on inputs from one or more pressure sensors coupled upstream and/or downstream of the GPF. If it is determined that the soot level on the GPF is lower than the target level for the current engine operating conditions, one or more engine actuators may be adjusted to actively accumulate soot on the GPF. As an example, a start of fuel injection timing may be advanced, and/or a fuel rail pressure may be reduced to increase soot levels in the exhaust stream based on the actual soot level relative to the target soot level. Also, an ash level on the GPF generated during prior regeneration events may be taken into account. For example, soot accumulation may be increased until a determined combined soot and ash level on the filter is at the target level. If it is determined that the current soot level on the GPF is higher than the target level for the current engine operating conditions, the GPF may be regenerated to remove the excess soot. A rate of the regeneration may be limited to reduce the soot level on the filter to the target level and not lower. Also, if the rate of regeneration is higher than a target rate, spark timing may be retarded to increase soot generation so that the soot level on the filter equilibrates to the target level at the end of the regeneration, and does not decrease below the target level.
In this way, by relying on soot and ash levels at a filter to increase the particulate matter (PM) capture rate of an exhaust system PM filter, the reliance on expensive filters having higher mesh density is reduced. By using filters with lower mesh density, exhaust backpressure may be reduced. As such, the reduction in backpressure increases engine power and fuel efficiency. The technical effect of maintaining a residual soot level (target level) at the GPF is that operating efficiency of the GPF may be improved. By actively adjusting the target level based on current engine operating conditions, performance of the exhaust emissions system may be optimized during all operating conditions including engine cold-starts. Overall, by using a lower filtration capability GPF and by maintaining a residual soot level on the GPF, engine efficiency, emissions quality, and fuel efficiency may be improved in a gasoline engine system. In addition, exhaust soot control and exhaust backpressure control can be achieved using a less expensive filter.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.